distribution of peroxy radicals throughout the lower troposphere.
Various datasets acquired from field campaigns were used to
evaluate the accuracy of the chemical scheme and dynamical core
that characterise the STOCHEM-CRI model. Good agreement between
model and measurement data was found for most of the
stations, but the low resolution of the model computed average
concentrations over each grid square of dimensions 5 latitude 5
longitude resulted in an under-prediction of model RO2 for some
stations in relation to the levels recorded in field campaigns. The
peak model RO2 is found in tropics because of higher emissions of
VOCs and rapid photochemistry. Both Northern and Southern
Hemisphere locations exhibited maximum peroxy radical concentrations
in the summer months when O3 photolysis is at its highest
and a minimum in winter when solar intensity is negligible. The
presence of RO2.H2O complexes has been found substantially in
tropical locations at 300 K where atmospheric moisture is in a high
abundance-approaching 10e12% of total peroxy radical concentrations.
The biogenically derived isoprene species-H2O complexes
could have a significant impact on the chemistry of their uncomplexed
counterparts in the troposphere. Complexation appears to
play a more significant role in the upper troposphere at sub
ambient temperature. Although the level of complexation is
modest, it is not certain what significant effects such complexation
will have on reaction rates and product branching ratios.